One way in which the resolution of a television picture can be increased is to convert an interlaced image into a progressive-scanned image. According to one technique disclosed in European patent application EP-A 0 192 292, this conversion is accomplished by examining pixels in three consecutive picture lines in consecutive first and second fields of the interlaced image signal, selecting a pair of pixels (P,Q), one pixel from each of two different lines in the second field, that most resemble one another, evaluating these two pixels and a third pixel (D) taken from the third line in the first field to find the median Y', and using this median to generate a new pixel Y in the second field. The technique works reasonably well for some images but generates artifacts such as a serration of sloping contours that move from frame to frame.
One attempt to solve this problem is disclosed in U.S Pat. No. 5,001,563. According to the technique disclosed therein, the median Y' is compared with the pixel D and, if the two values differ by more that some threshold amount, new pixel Y is formed from an average of the two pixels (P,Q). In one variation of this technique, new pixel Y is formed from a weighted sum of this average and the pixel (D). In yet another variation, if the best mutual resemblance of a pair of pixels is only slightly better than the worst mutual resemblance of another pair of pixels, then the pair of pixels that are vertically oriented with respect to the location for pixel Y are chosen as pixels (P,Q). Unfortunately, this technique generates objectionable artifacts in pictures where two or more pairs of pixels have high mutual resemblance.
Another interlaced-to-progressive-scan image conversion technique is disclosed in U.S. Pat. No. 5,532,751. According to this technique, variation between pixels in an image are evaluated to detect edges or contours. If the variation between pixels is below a threshold, the orientation of edge is deemed to have been successfully estimated and a new pixel is formed from the average of pixels lying along the estimated orientation. If the estimate of edge orientation is deemed to be unsuccessful, a new pixel is formed from the average of two vertically aligned pixels. This technique also generates objectionable artifacts in pictures having two or more pairs of pixels with high mutual resemblance.